1. Field of the Invention
This invention relates to a heating device for a fixing system for use in such image forming apparatuses as electrophotographic copying machines, printers, and facsimile systems and more particularly to a heating device for use in an induction heating type fixing system for thermally fusing a toner image on a sheet and fixed the toner image on the sheet.
2. Description of the Related Art
Electrophotographic copying machines and other similar apparatuses are provided with a fixing system. The fixing system thermally fuses a toner image transferred on a sheet such as a recording paper or a transfer material, a recording medium and fixed the toner image on the sheet. A halogen lamp heating method such as is used in a heat roller fixing system and a induction heating method such as is used in a film fixing system may be cited as concrete examples of the heating technique used by the heating device in the fixing system. In recent years, the induction heating technique has been attracting attention on account of the advantage that the rate of temperature rise is high.
The film fixing system with the excellent temperature rise characteristics as disclosed in JP-A-07-114,276 and JP-A-08-16, 007 is proposed concerning the conventional fixing systems adopting the induction heating method. The film fixing system is provided with a film as a rotator, an exciting coil disposed on the inner side of the film, and a pressure roller. The film fixing system generates the magnetic flux in the exciting coil to generate an eddy current in the film for induction heating. Then, it causes the sheet to move in concert with the heated film while heating and fusing the toner on the sheet for fixation. In the fixing system disclosed in the patent publications, the rate of temperature rise of the fixing system is heightened by heating only the nip part of the periphery of the film in contact with the pressure roller.
The conventional fixing system is so constructed as to heat the nip part only. Consequently, it must perform both the heating of the film as a rotator and the heat transfer from the film to the sheet within a very small span of time in which the sheet passes through the nip part. A low-speed copying machine or printer may take a relatively long time to pass one sheet through the nip part. It can heat the film in the nip part thoroughly and thus fulfills the desired function of fixing. However, the medium- to high-speed copying machine or printer has to take a relatively short time for the passage of one sheet through the nip part. Namely, it requires to move the sheet and the film at a high speed and is incapable of thoroughly heating the film in the nip part. And it has the possibility of suffering defective fixation particularly on the rear end of the sheet along the direction of conveyance.